Durability questions...

I'm considering building a cob house with as little outside bought material as possible. Both for financial reasons, and I really like the idea of creating a home from materials that come from my own land. My husband, however, isn't convinced. He's worried about how tough and waterproof it will be.
Can you water-mop an earthen floor? If something really greasy and nasty ends up on the floor, could I use a scrub brush or would that dig into the floor? What sort of soap would you use? I currently use Murphy's oil soap on my antique wood floors, would that work?
If the floor is properly tamped down, will dog nails scratch the floor? What about doggie 'accidents'? Or if someone drags their feet or wears spike heels? I understand it is easy to repair yourself, but I don't want that to be a new daily job...

Like many things, it depends.  The good news is earthen floors can be quite durable.  However, it depends on the mix of floor material and how you seal/finish it.  Easily damaged earthen floors usually have too little sand (too much clay) or weren't sealed and finished enough.

We have earthen floors with radiant heat in our house in all rooms except the bathroom and foyer.  The floor was sealed with linseed oil (with citrus thinner) and then two coats of a hardwood floor finish.

We wet mop the floor frequently (it does not absorb any water).  Greasy spills are no problem, though you don't want to leave a serious degreaser on the floor for too long since it will damage the hardwood finish.  A scrub brush would be o.k. as long as it has plastic or natural fiber bristles.  A metal wire brush would damage it.  Dog nails or normal shoes don't scratch.  Though boots with sand/gravel stuck in the treads will scratch.  Furniture with small, hard feet may make small dents.  Also, I dropped a dutch oven that made a small dent.  We have rugs under the rolling office chair and living room furniture to prevent wear.

My husband and I have lived on our earthen floor for a few years now, and have had almost the same experience as Ardilla.
The oil finishing process is so key.  Straight linseed oil on the first coat, and slowly more and more orange oil solvent added with additional layers.  Hardwood floor finish is an interesting idea.  I know many folks choose to wax also.
With adequate oil, though, the floor will actually NOT get scratched even with a wire brush.  I've tried.  The reason I tried is because I have had to patch the floor in a couple areas.
During periods of high humidity and hot weather (I'm in Arkansas) the floor will soften to the point that if a person sits in a chair with pointy legs for too long, the floor will dent.  When I tried to dig out the dent and abrade the adjacent floor, I found I could chisel through the oil layer, but nothing but nothing would abrade it.  It took the sand right off of sandpaper.  Quite impressive really.
I had thought if I could abrade the oil layer off of the area surrounding the patch, I'd be able to blend the new patch in better.  But instead, I settled for washing any cob smears off of the adjacent oiled floor and oiling the patch to match.  As long as I burnished a few times during the drying process, I was able to prevent any cracking between old and new and the result looks good to me.